Hydrogen chloride (\(\text{HCl}\)) dissolves in water readily. This molecule, which exists as a gas under normal conditions, has a strong attraction to water, resulting in high solubility. When hydrogen chloride gas is introduced into water, it reacts immediately to form a new substance known as hydrochloric acid.
The Nature of Hydrogen Chloride Gas
Hydrogen chloride is a colorless gas at room temperature with an irritating odor. As a pure substance, \(\text{HCl}\) exists as a diatomic molecule held together by a covalent bond.
The bond connecting the hydrogen and chlorine atoms is highly polar. Chlorine is significantly more electronegative than hydrogen, pulling the shared electrons closer to itself. This uneven sharing creates a dipole moment, giving chlorine a partial negative charge and hydrogen a partial positive charge. This polarity governs the molecule’s behavior when it encounters water.
Why Water Readily Absorbs HCl
The high solubility of hydrogen chloride results from the strong affinity between its polar molecule and the highly polar water molecule. Water molecules also possess a dipole moment, with oxygen carrying a partial negative charge and hydrogen carrying a partial positive charge. This allows water to effectively surround and interact with the \(\text{HCl}\) molecule.
When the substances mix, the attraction between the opposing charges is strong enough to break the covalent bond holding the hydrogen and chlorine together. This process is called ionization or dissociation, where the \(\text{HCl}\) molecule splits apart. The hydrogen atom forms a positively charged hydrogen ion (\(\text{H}^+\)) and the chlorine atom forms a negatively charged chloride ion (\(\text{Cl}^-\)).
The isolated hydrogen ion does not remain free in the solution. It rapidly bonds with a water molecule (\(\text{H}_2\text{O}\)) to form a hydronium ion (\(\text{H}_3\text{O}^+\)). This transfer of a proton from the hydrogen chloride to the water molecule occurs almost completely. The complete conversion of the \(\text{HCl}\) gas into these ions is the chemical reason for its high solubility and reactivity in an aqueous environment.
The Outcome: Creating a Strong Acid
The resulting aqueous solution, hydrochloric acid, is categorized as a strong acid due to the complete ionization process. Nearly 100 percent of the original hydrogen chloride molecules convert into hydronium ions and chloride ions when dissolved in water. The concentration of hydronium ions determines the acid’s strength.
Due to this high concentration of hydronium ions, hydrochloric acid solutions exhibit a low \(\text{pH}\) value. The \(\text{pH}\) scale is a logarithmic measure of acidity, where a lower number indicates a greater acidic nature. For example, gastric acid in the human stomach maintains a \(\text{pH}\) typically ranging between 1.5 and 3.5.
Hydrochloric acid plays a biological role as the main acidic component of gastric acid, helping activate digestive enzymes and kill harmful bacteria. Industrially, the solution is used in the pickling of steel to remove rust and impurities. It is also used in the manufacturing of organic compounds, such as producing vinyl chloride for \(\text{PVC}\) plastic, and is a component in many household cleaners.